Isosulfan blue, having a chemical name, N-[4-[[4-(diethyl amino)phenyl](2,5-disulfophenyl)methylene]-2,5-cyclohexadien-1-ylidene]-N-ethylethanaminium, sodium salt and the formula
is a triarylmethane dye used as a contrast agent for the delineation of lymphatic vessels and is particularly useful as a cancer diagnostic agent. Also known chemically as sulfan blue or patent blue, isosulfan blue is an active pharmaceutical ingredient used in the Lymphazurin™ blue dye pharmaceutical dosage form, available as 1% (10 mg/ml) 5 ml solution in phosphate buffer for injection. It is commonly used in a procedure called “mapping of the sentinel lymph nodes”. It is an adjunct to lymphography for visualization of the lymphatic system draining the region of injection. It has been used with increasing frequency in localizing sentinel lymph nodes in breast cancer patients. Isosulfan blue-guided surgical removal of cancerous tissue has been on the rise as it is cost effective and safer to use than technetium 99M radioisotope-labeled sulfur colloid. Isosulfan blue is a structural isomer of sulphan blue; both belong to the family of triarylmethane dyestuffs. Generally, preparation of triarylmethane dyes involves condensation of suitably substituted aryl aldehydes with 2 equivalents of alkyl-aryl amines giving rise to leuco-bases or leuco-acids followed by oxidation. Although the literature is replete with methods of preparing triarylmethane dyes, most of the methods involve strong acids for condensation resulting in leuco-bases or leuco-acids, hazardous oxidizing agents (lead oxide, chloranil, iron phthalocyanine/oxone) for converting to triarylmethane dyes, and crude methods (precipitation with sodium sulfate) of purification. See for example U.S. Pat. Nos. 4,330,476, 4,710,322, 1,531,507, 5,659,053, 1,805,925, 2,422,445, 1,878,530 and 2,726,252. Prior art methods of isolation of the crude leuco-acids or leuco-bases involve tedious neutralization/basification with strong bases and typically using the reaction mixtures in the oxidation step, giving rise to crude triarylmethane dyes. The triarylmethane dyestuffs thus prepared are used mainly for dyeing fabric, coloring paper, and printing inks. The literature cites utilization of the same aforementioned synthetic and isolation methods for the preparation of diagnostically important dyes, such as isosulfan blue, sulphan blue and patent blue V. See, Rodd's Chemistry of Carbon Compounds by S. Coffey, 1974 2nd Edition, Volume III Part F, 110-133.
Therefore there is a need in the art for an improved method in the process chemistry of isosulfan blue to be prepared in the purest form which is suitable for large scale cGMP production for its pharmaceutical formulation manufacturing.